Lambda repressor (l6-85) is a soluble fragment of lambda repressor containing residues 6 to 85 which maintains a native-like fold consisting largely of alpha helices. Differential scanning calorimetry (DSC) of the WT (l6-85) and the double mutant G46A/G48A reveal that the effect of the mutations is to stabilize the protein by ~ 1.5 kcal/mol at 25 C (I.e., 2.9 kcal/mol for the WT and 4.4 kcal/mol for G46/48A). The increase in stability upon mutation suggests that the primary effect of the mutation on stability is the decrease in conformational entropy ( Sconf) of the unfolded state when Gly is replaced by Ala. However, as the calculated effect of changing Gly to Ala at two positions in a helix is ~ 2.4 kcal/mol, the ~0.9 kcal/mol difference between the experimental and calculated values suggests that additional effects arise from the mutations which decrease the stability of the native state of the double mutant.